SNR Spectra as a Quantitative Model for Image Quality in Polychromatic X-Ray Imaging

In polychromatic x-ray imaging for nondestructive testing, material science or medical applications, image quality is usually a problem of detecting sample structure in noisy data. This problem is typically stated this way: As many photons as possible need to be detected to get a good image quality. We instead propose to use the concept of signal detection, which is more universal. In signal detection, it is the sample properties which are detected. Photons play the role of information carriers for the signal. Signal detection for example allows modeling the effects which polychromaticity has on image quality. $\mathit{SNR}$ spectra (= spatial $\mathit{SNR}$) are used as a quantity to describe if reliable signal detection is possible. They include modulation transfer and phase contrast in addition to noisiness effects. $\mathit{SNR}$ spectra can also be directly measured, which means that theoretical predictions can easily be tested. We investigate the effects of signal and noise superposition on the $\mathit{SNR}$ spectrum and show how selectively not detecting photons can increase the image quality

Comparison of different sources for laboratory X-ray microscopy

This paper describes the setup of two different solutions for laboratory X-ray microscopy working with geometric magnification. One setup uses thin-film transmission targets with an optimized tungsten-layer thickness and the electron gun and optics of an electron probe micro analyzer to generate a very small X-ray source. The other setup is based on a scanning electron microscope and uses microstructured reflection targets. We also describe the structuring process for these targets. In both cases we show that resolutions of 100 nm can be achieved. Also the possibilities of computed tomography for 3D imaging are explored and we show first imaging examples of high-absorption as well as low-absorption specimens to demonstrate the capabilities of the setups.Comment: 6 pages, 4 figures, proceedings of the 14th International Workshop on Radiation Imaging Detector

Comparing Image Quality in Phase Contrast subμ\mu X-Ray Tomography -- A Round-Robin Study

How to evaluate and compare image quality from different sub-micrometer (sub$\mu$) CT scans? A simple test phantom made of polymer microbeads is used for recording projection images as well as 13 CT scans in a number of commercial and non-commercial scanners. From the resulting CT images, signal and noise power spectra are modeled for estimating volume signal-to-noise ratios (3D SNR spectra). Using the same CT images, a time- and shape-independent transfer function (MTF) is computed for each scan, including phase contrast effects and image blur ($\mathrm{MTF_{blur}}$). The SNR spectra and MTF of the CT scans are compared to 2D SNR spectra of the projection images. In contrary to 2D SNR, volume SNR can be normalized with respect to the object's power spectrum, yielding detection effectiveness (DE) a new measure which reveals how technical differences as well as operator-choices strongly influence scan quality for a given measurement time. Using DE, both source-based and detector-based sub$\mu$ CT scanners can be studied and their scan quality can be compared. Future application of this work requires a particular scan acquisition scheme which will allow for measuring 3D signal-to-noise ratios, making the model fit for 3D noise power spectra obsolete

Characterization of Filigree Additively Manufactured NiTi Structures Using Micro Tomography and Micromechanical Testing for Metamaterial Material Models

This study focuses on the influence of additive manufacturing process strategies on the specimen geometry, porosity, microstructure and mechanical properties as well as their impacts on the design of metamaterials. Filigree additively manufactured NiTi specimens with diameters between 180 and 350 µm and a nominal composition of Ni50.9Ti49.1 (at %) were processed by laser powder bed fusion in a first step. Secondly, they structures were characterized by optical and electron microscopy as well as micro tomography to investigate the interrelations between the process parameters, specimen diameters and microstructure. Each specimen was finally tested in a micro tensile machine to acquire the mechanical performance. The process strategy had, besides the resulting specimen diameter, an impact on the microstructure (grain size) without negatively influencing its quality (porosity). All specimens revealed a superelastic response while the critical martensitic phase transition stress decreased with the applied vector length. As a conclusion, and since the design of programmable metamaterials relies on the accuracy of FEM simulations, precise and resource-efficient testing of filigree and complex structures remains an important part of creating a new type of metamaterials with locally adjusted material behavior

Modeling of negative Poisson’s ratio (auxetic) crystalline cellulose Iβ

Energy minimizations for unstretched and stretched cellulose models using an all-atom empirical force field (Molecular Mechanics) have been performed to investigate the mechanism for auxetic (negative Poisson’s ratio) response in crystalline cellulose Iβ from kraft cooked Norway spruce. An initial investigation to identify an appropriate force field led to a study of the structure and elastic constants from models employing the CVFF force field. Negative values of on-axis Poisson’s ratios nu31 and nu13 in the x1-x3 plane containing the chain direction (x3) were realized in energy minimizations employing a stress perpendicular to the hydrogen-bonded cellobiose sheets to simulate swelling in this direction due to the kraft cooking process. Energy minimizations of structural evolution due to stretching along the x3 chain direction of the ‘swollen’ (kraft cooked) model identified chain rotation about the chain axis combined with inextensible secondary bonds as the most likely mechanism for auxetic response

Roentgenbildgebung mit teilkohaerentem Synchrotronlicht: Anwendung auf Metalllegierungen, Zahnbein und Naturstein

Das harte Spektrum der BAMline am Berliner Synchrotron BESSY bietet die seltene Möglichkeit, Experimente mit hochauflösender Röntgenbildgebung und teil-kohärenter Strahlung zu machen. Diese Promotionsarbeit umfasst die Entwicklung eines neuen Tomographieaufbaus und dessen Anwendung auf drei spezifische Probleme der Materialwissenschaften aus den Gebieten der Werkstoffkunde, Biologie und Geologie. Die Mikrostruktur von Metalllegierungen nimmt eine Vielzahl geometrischer Formen an (runde Körper, Lamellen, Dendriten, etc.). Die Grenzschicht zwischen den einzelnen Phasen beschreibt diese Geometrie und sie verändert sich unter bestimmten Einflüssen, z. B. wenn eine Legierung in halbfestem Zustand verarbeitet wird. Dieser Zustand wird mit einem fest-flüssig Gemisch beschrieben welches bei teilweisem Schmelzen der Legierung entsteht. Sowohl statische Partikelreifung als auch äußere Scherkräfte führen zu einer Veränderung des Gefüges. Statische Röntgenbilder und Filmsequenzen wurden in 2D und 3D von einer Reihe von Aluminiumlegierungen gemacht. Anders als Oberflächenmessungen erlaubt Radiographie und besonders Tomographie eine statistische Analyse der 3D Gefügeparameter, z.B. Partikel-Größe, -Form, -Lage, -Koordinationszahl und Kontiguität. Reifungsgesetze für Agglomerate (bei hohem Festphasenanteil) und rheologische Eigenschaften wurden somit erforscht. Zahnbein ist das Grundmaterial unserer Zähne. Es liegt unter einer harten Schutzkappe aus Zahnschmelz. Zahnbein wird von Mikrometer-großen quasi-parallelen Kanälen durchzogen, denen im Allgemeinen eine mechanische Funktion zuschrieben wird. Trotz zahlreichen 2D Messungen und darauf aufbauenden Strukturmodellen, ist diese Funktionalität so unklar wie denn je. 3D Messungen sollen helfen diese Lücke zu schließen. Tomographie mit Hilfe von Fresnel-Beugung ist hervorragend geeignet, um die räumliche Anordnung der Kanäle zu verstehen. Mit dieser Arbeit wurde eine spezielle Technik entwickelt, mit der hochauflösende 3D Bilder von nassem Zahnbein aufgenommen werden können. Richtung, Verlauf und Biegung von einzelnen Kanälen können somit bestimmt werden. Simulationen der optischen Beugung an einem einzelnen Kanal zeigen ferner, dass die Beugungsbilder zusätzliche Informationen über die Dicke jener dichten Mineralschicht liefert, die die hohlen Kanäle umschließt (die Dicke kann aus dem Kontrast der Interferenzstreifen extrapoliert werden). Das Bruchverhalten von Steinen zu verstehen ist eine der grundlegendsten Aufgaben der Geologie. Trotz der sehr inhomogenen Mirkostruktur von Steinen entstehen Brüche nicht in willkürlichen Mustern sonder folgen einer bekannten „treppenförmigen“ Ausbreitung, welches ihren Ursprung in den Wechselwirkungen zwischen Zug- und Scherrissen hat. Mit Hilfe von Absorption- und Fresnel-Beugungs-Tomographie wurden Bilder von verschiedenen Gesteinen, vor und nach mechanischer Belastung erstellt. Zuerst wurden Grauwacke und Kalkstein als Vertreter von Sedimentgestein unterschiedlicher Korngrößen untersucht. Dann wurden Basalt und Granit verwendet, um unterschiedliche Gesteinstypen zu testen. In allen Steinen konnte Rissausbreitung beobachtet werden, die bei gesteigerter Belastung zum Bruch führte. Die Daten wurden mit einem bekannten mechanischen Modell verglichen, wobei Mikrosprünge das Entstehen von größeren Spalten begünstigen. Fresnel-Beugung erlaubte es zusätzlich, die Porosität und evtl. Erzminerale an den Korngrenzen besser zu erkennen. In Grauwacke wurde so eine Vorzugsrichtung für neu entstehende Risse gefunden.The hard spectrum which is available on the BAMline at Berlin’s synchrotron BESSY offers the rare opportunity to perform high-resolution X-ray imaging experiments with a partially coherent beam. This thesis work reports on the development of a new tomography system, including Fresnel-propagated imaging, and its application to three specific materials science problems from the fields of engineering materials, biology and earth science. The microstructure of metallic alloys exhibits a large variety of geometrical forms (globular, lamellar, dendritic, etc.). The interface between the different phases which characterizes this geometry, changes its shape under certain experimental conditions, in particular when the material is processed in the semisolid state. In this state the alloy is characterized by a solid-liquid mixture which is obtained by partial melting. Both static particle coarsening and external shearing forces are known to alter the microstructure of the alloy. In this work, static and dynamic 2D and 3D images were recorded from a variety of aluminum-based alloys. Unlike surface imaging methods, X-ray radiography and particularly tomography provide direct access to the 3D particle chararacteristics, e.g. size, shape, orientation, connectivity and contiguity. Coarsening of particle agglomerates (at high solid volume fraction) in liquid solution, as well as rheological properties of semi-solid alloys are thus characterized. Dentin is the bulk material of teeth, and is covered with a cap of dense enamel. Dentin is characterized by a quasi-parallel arrangement of micrometer-sized tubules which are believed to incarnate a natural mechanical design. Despite numerous measurements on 2D sections and structural models, the structure function relationship between tubules and tooth performance is still far from being fully understood which is why 3D maps of dentin microstructure were recorded. Fresnel-propagated tomography is shown to be the ideal technique to measure the arrangement of the tubules. This work shows how high-resolution 3D images of water-immersed tooth dentin are recorded, and detailed simulations of the optical wave propagation reveal that Fresnel-images contain additional information about the dense cuff of peritubular dentin surrounding the tubules. The cuff thickness can be extrapolated from the interference fringes that form the propagated images of tubules. Thus, 3D images are shown to contain far more information than the mere position and orientation of the individual tubules. Understanding rock fracture is one of the fundamental problems in earth science. Although rocks are an extremely inhomogeneous material, cracking does not occur randomly but it follows a well known en-echelon (staggered) propagation that results from mechanical interaction between tensile and shear-cracks. Absorption and Fresnel-propagated X-ray tomography are applied to measure samples of different rocks before and after mechanical compression nondestructively. In a first approach, limestone and greywacke are investigated, representing two sedimentary rocks of different grain size. Basalt and granite are tested in a second approach to compare different rock types. Development of cracks is observed in all materials, leading to fracture when increasing mechanical load is applied. In this work, relatively small mm-sized samples are used in order to test a classical fracture model wherein micro-flaws initiate the formation of larger cracks. For the first time, Fresnel-propagated imaging is applied to rock samples, highlighting micrometer-sized intergranular porosity as well as different material phases. The latter is shown to indicate a preferred crack orientation

Phase-Contrast and Dark-Field Imaging

Very early, in 1896, Wilhelm Conrad Röntgen, the founding father of X-rays, attempted to measure diffraction and refraction by this new kind of radiation, in vain. Only 70 years later, these effects were measured by Ulrich Bonse and Michael Hart who used them to make full-field images of biological specimen, coining the term phase-contrast imaging. Yet, another 30 years passed until the Talbot effect was rediscovered for X-radiation, giving rise to a micrograting based interferometer, replacing the Bonse–Hart interferometer, which relied on a set of four Laue-crystals for beam splitting and interference. By merging the Lau-interferometer with this Talbot-interferometer, another ten years later, measuring X-ray refraction and X-ray scattering full-field and in cm-sized objects (as Röntgen had attempted 110 years earlier) became feasible in every X-ray laboratory around the world. Today, now that another twelve years have passed and we are approaching the 125th jubilee of Röntgen’s discovery, neither Laue-crystals nor microgratings are a necessity for sensing refraction and scattering by X-rays. Cardboard, steel wool, and sandpaper are sufficient for extracting these contrasts from transmission images, using the latest image reconstruction algorithms. This advancement and the ever rising number of applications for phase-contrast and dark-field imaging prove to what degree our understanding of imaging physics as well as signal processing have advanced since the advent of X-ray physics, in particular during the past two decades. The discovery of the electron, as well as the development of electron imaging technology, has accompanied X-ray physics closely along its path, both modalities exploring the applications of new dark-field contrast mechanisms these days. Materials science, life science, archeology, non-destructive testing, and medicine are the key faculties which have already integrated these new imaging devices, using their contrast mechanisms in full. This special issue “Phase-Contrast and Dark-field Imaging” gives us a broad yet very to-the-point glimpse of research and development which are currently taking place in this very active field. We find reviews, applications reports, and methodological papers of very high quality from various groups, most of which operate X-ray scanners which comprise these new imaging modalities